Method of producing rare earth metals and silicon alloys



April 22k, 1969 `R. KALLENBACH ET A1. 3,440,041

METHOD 0F PRODUCING RARE` EARTH METALS AND SILICON ALLOYS Filed May e. i966 w lll' D IIIIIIII 4 4 Q S GARDT LENBACH INVENTO WALTER BUN RUDOLF KAL MfS/m l ATTORNEYS United States Patent O U.S. Cl. 75--152 4 Claims ABSTRACT F THE DISCLOSURE Master alloys of rare earth metals and silicon are yprepared by forming a reaction mixture containing between about 20 to 45 percent by weight of rare earth metal oxides or salts, between about to 65 percent by weight of silicon or a ferrosilicon alloy containing at least 30 percent by weight of silicon, and between about 15 to 40 percent by weight calcium carbide, the reaction mixture being heated to effect reduction of the rare earth metal constituent of the mixture and to obtain a rare earth metal and silicon alloy product containing between about 25-35 percent by Weight silicon and between about 45-65 percent `by weight of rare earth metals plus iron.

This invention relates to the alloys of the rare earth metals and silicon, and more particularly to the production of Such alloys containing between about 25 to 35% by weight silicon and between about 45 and 65% by weight iron plus rare earths by the pyrometallurgical reduction of rare earth metal oxides and salts in the presence of alloys of silicon and ferrosilicon.

The rare earth metals are the 15 elements of the lanthanide series having atomic numbers 57 to 71 inclusive, although the element yttriurn (atomic number 39) is commonly found with and included in this group of metals. The most common alloy of the rare earth metals which contains the metals in the approximate ratio in which they occur in their most common naturally occurring ores is known as misch metal, and intermetallic compounds of rare earth metals and silicon are known as misch metal silicides.

The rare earth metals, and particularly cerium which is the most plentiful of these metals, are valuable alloying additives for improving the metallurgical properties of alloyed and unalloyed steel, cast iron and other metals. However, as the rare earth metals exhibit a great tendency to be oxidized particularly at high temperatures, the direct addition of these metals (for example, as misch metal) to molten iron and steel ordinarily results in excessively high loss of the rare earth metals. As a result, the rare earth metals are ordinarily added to the molten metal in the form of silicon alloys or silicides containing a relatively small proportion of rare earth metals. Typical master alloys might contain from 35 to 50% silicon, up to 15% rare earth metals, and the remainder such metals as calcium, iron, aluminum and the like.

Silicon or silicon and iron alloys (hereinafter referred to as ferrosilicon alloys) containing very small quantities of the rare earth metals can be produced by introducing the oxides or salts of the rare earth metals directly into melts of silicon or ferrosilicon base alloys. The rare earth metal content of the alloys thus obtained ranges from 0.1 to 1.23%. Moreover, as the iron content of the alloy increases the yield of rare earth metals, or the quantity of the rare earth -metals present in the alloy in relation to the quantity of rare earth metal oxides added to the melt, decreases from about 66 to 10%.

German Patent 1,131,417 describes an improvement in the aforementioned procedure wherein the addition of the rare earth metal oxide or salt to the silicon or ferrosilicon base alloy is carried out in the presence of a calcium oxide-containing calcium iiuoride flux. The reduction of the rare earth metal oxides or salts with said silicon or ferrosilicon is carried out in a carbon crucible in an electric arc furnace employing carbon electrodes, and silicon and ferrosilicon alloy products containing as much as 28% by weight of rare earth metals have been obtained by the process. However, the process is unsatisfactory due to the highly corrosive character of the ux employed, to the technical ditliculties inherent in electric arc furnace operations employing such highly reactive materials as the highly basic lime-iluorite flux, to the unpredictable behavior of the reaction mixture and in particular to the evolution of toxic fumes resulting from the decomposition of the basic lime-tiuorite flux in the electric arc, to the health hazard created by the evolution of such fumes, and to the diiiiculty in obtaining a uniform and reproducible product by means of the process.

The present invention relates to an improved process for producing master alloys of the rare earth metals and silicon or rare earth metals and ferrosilicon which is devoid of technical diiculties and which results uniformly in an alloy product containing between about 25 and 35% by weight of silicon and between about 45 and 65% by weight of iron plus rare earth metals (that is, the combined weight of the iron and rare earth metal constituents of the alloy is between about 45 and 65% of the total weight of the alloy product). The process is based on the discovery that if calcium carbide is added to the initial reaction mixture, and if the reaction mixture of rare earth metal oxides or salts, silicon or ferrosilicon alloys and calcium carbide contains predetermined quantities of these three constituents, a rare earth metal-silicon or rare earth metal-ferrosilicon alloy product containing silicon, rare earth metals and iron in the proportions mentioned above can be produced without difficulty and with a high degree of reproducibility.

Accordingly, pursuant to the present invention the irnproved process comprises forming a reaction mixture containing between 20 and 45% by weight, and preferably between about 20 and 40% by weight, rare earth metal oxides or the equivalent thereof contained in rare earth metal salts, between about 15 and 65 by weight, and preferably between about 25 and 65 by weight, of silicon present in the form of a silicon or ferrosilicon alloy containing at least 30% by weight of silicon, and between about 15 and 40% by weight, and preferably between about l5 and 35% by weight, calcium carbide. The reaction mixture is heated to a temperature sufficient to effect the reduction of the rare earth metal oxides or salts and to alloy these metal-s with the silicon or ferrosilicon alloy, and preferably to a temperature within the range of 1200 to 1800 C. The master alloy obtained as a result of this process is a substantially uniform product containing between about 25 and 35% by weight of silicon, between about 45 and 65 by weight, collectively, of iron and rare earth metals, and up to about 10% by weight calcium. Advantageously the reaction is carried out in the presence of a flux which absorbs the slaggy secondary products of the reaction (e.g., silicon carbide), the flux preferably comprising one or more chlorides or fluorides of the alkaline earth metals.

The relative proportions of the mixed rare earth metal oxides, silicon in the form of a silicon or ferrosilicon alloy and calcium carbide present in the initial reaction mixture should fall within the area defined by the points A, B, C and D plotted on the triangular coordinate diagram comprising the single figure of the accompanying drawing, and in a particularly advantageous embodiment of the process the relative proportions of the three con- 3 stituents of the reaction mixture fall within the area defined by the points A, B', C and D plotted on the aforesaid diagram. The actual values of the points A, B, C and D, and A', B', C and D are set forth in the following table.

Rare earth Silicon as Si or CaCz metal oxides Fe-Si The silicon and ferrosilicon alloys employed in the practice of the process are of commercial grade, typical commercial silicon containing about 98% Si and typical commercial ferrosilicon containing from 40 to 50% by weight Si and the balance mainly iron being successfully employed in the practice of the process. Similarly, the calcium carbide employed in the process is advantageously of commercial or technical grade, a typical calcium carbide starting material employed herein containing about 80% CaC2 with the balance comprising mainly carbon and metallic impurities.

The composition of the master alloy product obtained by the practice of the present invention is to a high degree independent of the quantity of silicon, iron and rare earth metals present in the initial reaction mixture. That is to say, the alloy product will contain a substantially uniform content of silicon comprising about 30 *:5% by weight of the alloy product and a substantially uniform content of iron plus rare earth metals comprising about 55i10% of the alloy product, provided, of course, the composition of the initial reaction mixture is as specified herein. Moreover, the master alloy product of the process contains calcium in quantities up to by weight of the alloy product, which is an advantage in that it is sometimes desirable for the rare earth metal-silicon or rare earth metal-ferrosilicon alloy to contain calcium in order to obtain certain desired metallurgical effects.

The impurities present in the silicon and ferrosilicon alloy and the calcium carbide starting materials appear for the most part in the slag or in the alkaline earth metal halide flux, although the desired rare earth metal-silicon or rare earth metal-ferrosilicon alloy product of the process may contain insignificant amounts of other materials as impurities therein. The silicon content of the initial reaction mixture in excess of the quantity of silicon present in the master alloy product is to be found in the slag, chiey in the form of silicon carbide which can be separated therefrom as a valuable by-product of the process. The iron content of the master alloy product of the process increases the specic gravity of the alloy and thus facilitates the addition of the alloy to molten steel or iron, thereby reducing the loss of rare earth metals. Moreover, the fact that the silicon content of the master alloy product is low constitutes a special advantage when the alloy is employed as an additive for iron and steel alloys.

The following examples are illustrative but not limitative of the practice of the invention.

EXAMPLE I A reaction mixture was prepared having the following composition: 85 gm. commercial mixed rare earth metal oxides containing about 99% rare earth metal oxides, 105 gm. commercial silicon containing about 98% Si and the remainder mainly iron, and 110 gm. commercial calcium carbide containing about 80% CaCZ. The reaction mixture was admixed with 25 gm. of calcium fluoride as a flux and was heated in a carbon crucible to a temperature of about 1650 C. to effect reduction of the rare earth metal oxides and formation of the desired rare earth metal-silicon alloy. On completion of the reaction the metal alloy was separated from the slag and 116 gm. of an alloy product having a specific gravity of 4.03 was recovered. The master alloy product contained 46.0% misch metal, 33.4% silicon, 2.8% iron, 8.8% calcium and the remainder aluminum, carbon and other impurities.

EXAMPLE II A reaction mixture was prepared having the following composition: gm. commercial mixed rare earth metal oxides containing about 99% rare earth metal oxides, 120 gm. commercial silicon containing about 98% Si, and 90 gm. commercial calcium carbide containing about 80% CaC2. The reaction mixture was admixed with 75 gm. calcium fluoride as a flux and was heated in a carbon crucible at about 1650 C. to effect reduction of the rare earth metal oxides and to produce the desired rare earth metal-silicon master alloy product. After separation from the slag, a master alloy product weighing grams and having a specific gravity of 3.85 was recovered. The metal alloy product `contained 44% misch metal, 37.7% silicon, 5% iron, 6.9% calcium and the remainder aluminum, carbon and other impurities.

EXAMPLE III A reaction mixture was prepared having the following composition: 14.2 gm. commercial mixed rare earth metal oxides containing about 99% rare earth metal oxides, 34.2 gm. commercial ferrosilicon alloy containing about 50% Si and the remainder predominantly iron, and 11.6 gm. commercial calcium carbide containing about 80% CaC2. The reaction mixture was admixed with 15 gm. of calcium fluoride as a flux and was heated in a carbon crucible to a temperature of about 1600 C. to effect reduction of the rare earth metal oxides and to obtain the desired rare earth metal-ferrosilicon alloy product. After separation from the slag, a master alloy product weighing 31.7 gm. and having a specific gravity of 5.42 was recovered. The metal alloy product contained 27% misch metal, 31.5% silicon, 38% iron, 3.1% calcium and the remainder aluminum, carbon and other impurities.

EXAMPLE IV A reaction mixture was prepared having the following composition: 9.6 gm. commercial mixed rare earth metal oxide containing about 99% rare earth metal oxides, 40.2 gm. commercial ferrosilicon alloy containing about 50% Si and the remainder predominantly iron, and 10.2 gm. `commercial calcium carbide containing about 80% CaCg. The reaction mixture was admixed with 15 gm. of calcium uoride as a ux and was heated in a carbon Crucible to a temperature of about 1600 C. to effect reduction of the rare earth metal oxides and to obtain the desired rare earth metal-ferrosilicon alloy product. After separation from the slag, a master alloy product weighing 34.4 gm. and having a specific gravity of 5.22 was recovered. The metal alloy product contained 18% misch metal, 34% silicon, 43.5% iron, 1.6% calcium and the remainder aluminum, carbon and other impurities.

EXAMPLE V A reaction mixture was prepared having the following composition: 45 gm. commercial rare earth metal oxides containing about 99% of said oxides, 201 gm. commercial ferrosilicon alloy containing about 42% Si and the remainder predominantly iron, and 54 gm. commercial calcium carbide containing about 80% CaC2. The reaction mixture was admixed with 75 gm. of magnesium fiuoride as a flux and was heated in a carbon Crucible to a temperature of about 1600 C. to effect reduction of the rare earth metal oxides and to obtain the desired rare earth metal-ferrosilicon alloy product. After separation from the slag, a master alloy product weighing 209 gm. and having a specific gravity of 5.5 was recovered. The metal alloy product contained 13.7% misch metal, 34.8% silicon, 47.9% iron, 1.4% calcium and the remainder aluminum, carbon and other impurities.

EXAMPLE VI A reaction mixture was prepared having the following composition: 10.6 gm. commercial mixed rare earth metal oxides containing about 99% rare earth metal oxides, 41.2 gm. commercial ferrosilicon alloy containing about 42% Si and the remainder predominantly iron, and 8.2 gm. commercial calcium carbide containing about 80% CaC2. The reaction mixture was admixed with gm. of calcium fluoride as a ux and was heated in a carbon crucible to a temperature of about 1600 C. to effect reduction of the rare earth metal oxides and to obtain the desired rare earth metal-ferrosilicon alloy product. After separation from the slag a metal alloy product weighing 40.9 gm. and having a specic gravity of 5.58 was recovered. The metal alloy product contained 14% misch metal, 33.5% silicon, 51% iron, 0.2% calcium and the remainder aluminum, carbon and other impurities.

We claim:

1. Process for the production of master alloys of the rare earth metals and silicon which comprises:

forming a reaction mixture consisting essentially of between about and 45% by weight of mixed rare earth metal oxides selected from the group consisting of rare earth metal oxides and rare earth metal salts (calculated as equivalent oxides), between about 15 and 65% by Weight of silicon present in the form of at least one silicon alloy selected from the group consisting of silicon and ferrosilicon alloys containing at least by weight of silicon, and between about 15 and 40% by weight of calcium carbide, heating said reaction mixture to effect reduction of the rare earth metal constituent of the mixture and to obtain a rare earth metal-silicon alloy product containing between about 25 and 35% by weight silicon, between about and 65% by weight of iron plus rare earth metals, and up to about 10% by weight calcium, and

recovering the desired rare earth metal-silicon master alloy product.

2. The process according to claim 1 in which the reaction ymixture comprises between about 20 to 40% by weight of said rare earth metal oxides, between about 25 and by weight of silicon and between about 15 and 35% b"y weight of calcium carbide.

3. The process according to claim 1 in which the reaction mixture is heated to a temperature of between about 1200 and 1800 C. to effect reduction of the rare earth metal oxide and to obtain the desired rare earth metalsilico-Ir master alloy product.

4. The process according to claim 1 in which the reaction is carried out in the presence of a ux comprising at least one alkaline earth metal compound selected from the group consisting of alkaline earth metal uoridcs and alkaline e'arth metal chlorides.

References Cited UNITED STATES PATENTS 3,211,549 10/1965 Kusaka 75-134 3,295,963 1/1967 Galvin 75-152 3,364,015 1/1968 Sump 75-152 RICHARD O. DEAN, Primary Examiner.

U.S. Cl. X.R. -122, 123 134 

